Palynological records of the last 11,000 years in China

The Holocene vegetation in the Yangtze Delta is sensitive to global/regional climate change, sea-level oscillations, and human impacts, and it represents a valuable archive for deciphering environmental change. In this paper, the vegetation history of the northern Yangtze deltaic coast, and its response to climate/sea-level changes over the past 10.3 ka, have been investigated based on palynological assemblages from a well-dated sediment core (YZ07) in the northern Yangtze delta. Comparison of vegetation successions with regional climatic proxies, relative sea level change, and depositional environments suggests that this region was dominated by mixed subtropical and temperate forests in a warm /wet climate regime during the early Holocene (~10.3–8.7 ka), when it was characterized by tidally-influenced fluvial environments. From ~8.7 ka to 7.5 ka, subtropical and warm temperate pollen taxa decreased significantly while cool-tolerant conifers and grasses became more dominant, indicating a cooler and drier climate. During the mid-Holocene thermal optimum (~7.5–4.3 ka), mixed subtropical and temperate forests expanded markedly in coastal regions, caused by a return to a warm/wet climate during an episode of rising sea-level. At the start of late Holocene (~4.3 to 3.4 ka), subtropical and temperate trees were widely replaced by grasses around coastal environments, indicating a cooling climate. From ~3.4–1.7 ka, mixed subtropical and temperate forests flourished in the delta region, suggesting that climate returned to warm and wet conditions again. From ~1.7–1.3 ka, in delta environments, subtropical and temperate forests contracted under cool/dry climatic conditions. Since ~1.3 ka, subtropical and temperate pollen taxa increased around the marsh to intertidal flat environments, suggesting a warm/wet climate resurgence. The generally warm/wet Holoence climate of the Yangtze Delta region was interrupted by a series of short-term cold events at 8.2 ka, 7.3 ka, 4.2 ka, 3.5 ka and 1.4 ka. These fluctuations are broadly synchronous with regional climate variability and global sea-level oscillations. In addition, the influence of human cultivation on vegetation is recognized since the mid-Holocene (around 5.6 ka), with the appearance of Fagopyrum pollen in the northern Yangtze Delta.

Understanding how terrestrial carbon storage feeds back on warm climate states is critical for improving global warming projections. Soils may act as a positive feedback on climate if warming increases soil carbon decomposition rates. Conversely, if increases in net primary production (NPP) exceed increases in decomposition, the climate feedback will be negative. Here we utilize the first palaeoclimatic application of compound-specific ${\delta }^{13}$C measurements on n-fatty acid biomarkers (extracted from a stalagmite from central China) to constrain the response of catchment terrestrial carbon cycle feedbacks during warmer phases of the Holocene. We resolve proportional increases in C₃ plants in the catchment area during these warmer/wetter intervals. Moreover, we infer that heterotrophic soil respiration was highly substrate selective, indicating that NPP outpaced decomposition and the catchment behaved as a carbon sink (mediated and enhanced by changes in the relative proportion of C₃ vs. C₄ plants). Thus, we provide palaeoclimate evidence that subtropical soils in a warmer/wetter climate acted as a sink for organic carbon, and thus as a negative climate feedback, during warmer climatic phases.

Understanding the evolution of the Holocene East Asian summer monsoon (EASM) and vegetation response regimes can help predict the climatic variability and vegetation change of future warming. Here, we reconstructed Holocene vegetation and climate change based on pollen, grain size and percent organic matter from a lake sediment profile in Huitengxile Plateau of central Inner Mongolia, northern China. We found that temperate deciduous forest steppe and temperate mixed forest steppe prevailed under maximum EASM during the middle Holocene (~8.3–5.9 cal. kyr BP), with more open vegetation and a weak EASM prior to and following this period. The vegetation succession is consistent with regional records, suggesting a regional vegetation pattern, i.e., oak forest replacing birch and elm forest during the middle Holocene and pine forest expanding at the expense of birch and oak forest since ~6.0 cal. kyr BP. A regional synthesis suggests the maximum EASM began at 8.3–7.8 cal. kyr BP in northern China, indicating a significant delay when compared with published records from southern China. The maximum EASM in northern China ended at 5.9–5.5 cal. kyr BP, being similar or slightly delayed from that in southern China.

The coastal deltaic plain of the Yangtze River between Taihu (Lake Tai) and Shanghai in eastern China has been the scene of human settlement and agriculture since the early Neolithic, becoming increasingly intensive in the Upper Holocene when delta accretion and the establishment of a stable hydrological regime of freshwater lakes and wetlands allowed the development of extensive agriculture and complex society in late prehistoric and dynastic times. During this period the area was significantly affected by changes in sea level, climate and vegetation, resulting in a dynamic and complex environmental history, however little research has concentrated on environmental change and human impacts during the last few millennia. This study focuses on this late period, presenting the results of integrated sedimentary, microfossil and radiocarbon analyses from a core near the eastern margin of Lake Dianshan, to the east of Taihu. After the withdrawal of intertidal conditions and the conversion to freshwater lake at the core site about 2600 cal. yr BP, pollen and algal spore data show that increased sedimentation gradually reduced freshwater depth until a surface peat formed ca. 1500 cal. yr BP. This also dates the start of a switch in woodlands from sub-tropical and warm temperate trees to a mainly cool temperate and coniferous tree flora, under climatic cooling and human impact. After this time water depths at the site increased greatly, partly due to climate change and flooding, but also because of the establishment of deepwater ‘paddy’ agriculture. Microcharcoal and pollen data show that a major episode of human impact using fire, with deforestation and rice cultivation, occurred between ca. 1500 and 1200 cal. yr BP. These dates suggest it is one of the latest examples of ‘flooded-field’ ‘paddy’ cultivation before more intensive agricultural techniques were adopted in the area after ca. 800 cal. yr BP.

Lake Sihailongwan in Jilin province, NE China, provides the first continuous and almost entirely seasonally laminated sediment record on the East Asian mainland comprising the complete Holocene, the Late-glacial period, and large parts of the Last Glacial. Sediment and palynological proxy data provide a finely resolved regional environmental history of the East Asian monsoon. A varve-based chronology (shl-vc2) has been established for the last 65,000 years and allows a detailed comparison with other long regional and global palaeoclimate records. Vegetation density of the study area depends, on the long run, on precessionally forced insolation changes, with superimposed millennial-scale variability during the Last Glacial. Periodic increase of organic carbon content and thermophilous tree species like Ulmus and Fraxinus and contemporary decrease of shrub Alnus precisely mirror millennial-scale climatic variations primarily known from Greenland ice-cores as Dansgaard-Oeschger cycles, as well as Late-glacial period climate changes. Percentages of trees & shrubs pollen and in particular lake productivity-related data reveal substantial differences between interstadial intensities, with those between 50 and 60 ka BP being more pronounced than the following ones.

Phylogeography places population genetics in an explicitly spatial context, and in doing so attempts to reconstruct the historical and contemporary evolutionary processes acting across a species range through space and time. Here we present the phylogeographical structure of Daphnia magna as determined for full mitochondrial genomes from samples of 60 populations throughout much of the species known range, including Europe, the Middle East, and Asia. Contrary to previous analyses, the present analysis of the mitochondrial genome reveals coarse-grained (continental scale) evidence for spatial structure, and in particular a deep split between Western Eurasia and East Asian D. magna lineages. In contrast to previous analyses with nuclear genetic markers, our mitogenomic analysis reveals much less structure within lineages. We quantify divergence between species using the full mitochondrial genome sequence of a closely related species, D. similis. The distribution of European and Middle Eastern genetic diversity is consistent with a rapid demographic expansion following the end of the most recent ice age about 10,000 years before present. By estimating species wide distributions of d_N/d_S in mtDNA, we provide evidence that the effectiveness of purifying selection on protein coding genes in the mitochondrial genome of coastal rock pool populations, which have pronounced extinction-colonization dynamics, is reduced compared to larger and more stable non-rock pool populations. The present study adds important insights into the evolutionary history of a widely used model organism in ecology, evolution and ecotoxicology, and highlights the utility of phylogeographic analysis of organellar genomes to understand evolutionary processes.